1. Field of the Invention
The present invention relates to voice messaging systems. More particularly, it relates to voice messaging systems wherein the speech coding used to code incoming voice messages is changeable based on incoming call related information.
2. Description of Related Art
Voice messaging has become an everyday requirement in today's society. Early voice messaging apparatus comprised magnetic cassette tapes which recorded a significant amount of voice messages, e.g., 60 minutes of voice messages. However, cassette tapes were disadvantageous because of the mechanics and time required to fast-forward and rewind the cassette tapes to the storage points of individual messages.
More recently, particularly as the size of memory has increased in density while at the same time decreased in price, digital voice messaging systems have become commonplace. Digital voice messaging systems store incoming voice messages in digital memory, reducing the mechanics and cost, and increasing the reliability over conventional analog voice messaging systems.
It is commonly known to encode voice messages for storage in digital voice messaging systems to maximize the length of voice messages which may be stored in a finite amount of memory. Various conventional voice compression techniques exist, many having various effective data rates. For instance, linear predictive coding (LPC), code-excited linear predictive (CELP) coding, RPE-LTP, or adaptive differential pulse code modulation (ADPCM) are examples of techniques, any one of which may be utilized by conventional digital voice messaging systems, and any one of which may be implemented at varying effective data rates. These coding techniques result in a compression of the data yielding a reduction in the data rate necessary to represent the underlying voice message. The effective data rate resulting from a general linear approach to digitizing toll quality speech (e.g., 8 KHz, 8-bit sampling, i.e., 64 Kb/s) with respect to an effective data rate resulting from a coding technique such as CELP, LPC, or ADPCM forms a compression ratio of the coding. These compression techniques have been developed for use with digital voice messaging systems which in general reduce the number of bits necessary to represent voice message data. Although it is recognized that the compression techniques are improved over time to better approach toll quality speech, in general the greater the compression ratio, the greater the degradation in the quality of the speech stored in voice memory. "Compression ratio" as used herein refers to the ratio of the number of bits representing the voice message in linear form divided by the number of bits used to represent the same voice message in compressed or coded form.
FIG. 6 shows a prior art voice messaging apparatus such as a digital telephone answering device (TAD) 11. Although described with respect to embodiments relating to a TAD, the present invention is equally applicable to a voice mail type system and voice messaging systems in general.
In FIG. 6, a signal from a microphone or other analog signal source 106 is input to a codec, analog-to-digital converter, or other digitizing device in a voice recorder/playback module 20 for conversion to m-law or A-law pulse code modulated (PCM) data. The m-law or A-law PCM data is encoded and compressed by a variable compression ratio encoding routine in the processor 18. For playback, a variable compression ratio decoding routing in the processor 18 together with a codec, digital-to-analog converter, or similar device in the voice recorder/playback module 20 converts voice message data retrieved from voice memory into an analog signal for output to speaker 108.
While control of the TAD 11 and handling of voice processing is shown in FIG. 6 as being handled in processor 18, a separate digital signal processor may be utilized to handle the compression and other voice processing tasks. The processor 18 controls the encoding and decoding tasks performing the compression and decompression of the voice message data. Processor 18 also controls storage of the compressed (encoded) speech data into voice memory 100, and controls the retrieval of compressed speech data from the voice memory 100.
The TAD 11 further includes an alpha-numeric keypad 36 and display 27 as are known in the art. The display 27 displays call related information regarding an incoming call, e.g., a caller's telephone number and household name, received from a central office 13 via a telephone line interface 48 and call related information detector/receiver 12.
However, conventional voice messaging systems are limited by the compression rate used and the amount of available message memory. Messages which are received after the memory becomes full, such as after a long vacation, are conventionally refused.
Thus, there is a need to maximize the available message memory for use beyond that which is conventionally available such that more messages may be stored.